Hidden pain under the ribs in the left armpit may be lung diseases, pleurisy lesions, coronary heart disease and other diseases. 1. Lung diseases: Lung diseases such as upper respiratory tract infection or lung infection can cause pain due to the close proximity of the infected foci to the chest wall and the involvement of the foci in the pleura. 2. Pleural lesions: pleural lesions such as pleurisy, pleural tuberculosis, pleural tumors, etc. can produce chest pain, cough, chest tightness, shortness of breath and other symptoms due to the stimulation of the pleura by the lesion. 3. Coronary heart disease: when coronary heart disease attacks, it often manifests as pain in the left side of the chest wall, chest tightness, palpitation, shortness of breath and other symptoms, which can be radiated to the left armpit. Do not take painkillers on your own when you have pain, so as not to cover up your condition with the influence of drugs. Patients should go to the hospital in time for chest radiographs and other necessary examinations, so that the doctor can clarify the cause of the disease and carry out appropriate treatment.